Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted, either directly or through the use of a gearbox, to a main generator through a rotor shaft. This way, the main generator produces electricity which is to be supplied into the electrical grid.
Wind turbines may comprise pitch systems that are employed for adapting the position of the blades to varying wind conditions by rotating each blade along its longitudinal axis. Wind turbines may also comprise load sensors on the blades for measuring loads on the blades caused by for example the wind and/or the weight of the blades.
Too high loads on the blades can e.g. damage the blades and/or cause undesirable rotational speeds of the rotor which may damage other components of the wind turbine. The blade load sensors permit detecting high loads and make it possible to react, by e.g. acting on the pitch systems in such a way that loads on the blades may be reduced. These adjustments on the blades through the pitch systems may extend the life of the wind turbine and/or reduce the cost of producing power.
Blade load sensors can be calibrated in order to preserve their accuracy when taking measurements of the loads on the blades. Calibration normally comprises establishing correspondence between indications generated by the blade load sensors and values of reference according to calibration patterns (i.e. particular conditions for calibration). If the calibration process produces some inconsistency, suitable adjustments may be undertaken on the load sensors to improve their accuracy.
It is known that blade load sensors can be manually calibrated in a factory by e.g. statically pulling the blades to obtain particular conditions for calibration. This manual calibration is normally performed before mounting the blades on the wind turbine. However, over time, load sensors may need to be recalibrated.
It is also known that blade load sensors can be manually calibrated when the blades are mounted on the wind turbine by manually (i.e. mechanically) acting on the wind turbine to e.g. set the blade in a particular position (e.g. horizontal position) with a particular pitch angle. This manual calibration permits recalibrating the load sensors regularly. However this type of calibration may take a long time and may be especially expensive for offshore wind turbines because operators need to go where the wind turbine is located.
It is also known to use automatic calibration of blade load sensors during operation of the wind turbine by recording several minutes of data (or indications or load measurements) from the blade load sensors. For example, data from the load sensors may be recorded e.g. when predetermined conditions for calibration are met during idle operation of the wind turbine at low winds. Some of said predetermined conditions may be obtained after several hours or days of idling operation of the wind turbine. Therefore, a drawback of this type of calibration may be that it can take a long time, depending on the wind conditions.
For example, the international patent application WO 2011/092032 A1 discloses a calibration method of the type commented in the previous paragraph. This method comprises the steps of: a) determining a rotor azimuth angle, b) determining a pitch angle of a first wind turbine blade, c) measuring loads in a first cross-section of the first wind turbine blade using a first load sensors, d) calculating theoretical loads based on at least the rotor azimuth angle and the pitch angle of the blade determined in steps a) and b), e) comparing the loads measured in step c) with the theoretical loads calculated in step d), and f) calibrating the first load sensors based on the comparison of step e).
The method of WO 2011/092032 A1 allows calibrating the sensors onsite while the blades are mounted on the turbine, i.e. without needing to dismount the blades from the turbine. The calibration may be performed while the turbine is running and connected to the grid. A system is continuously logging the relevant data (turbine operating conditions and sensor measurements), and after some time it has collected enough data to be able to do a reliable calibration of the sensors. The calibration is done fully automatic. However, as argued before, execution of this method can take a long time.